Rarely Observed Phase Transitions in a Novel Lyotropic Liquid Crystal System

This Letter presents neutron diffraction data from a novel, biologically relevant, lyotropic membrane system which is highly alignable ( $\le 1.0°$ mosaic) in a magnetic field and gives rise to a number of well-defined Bragg reflections. The system, composed of two different phosphorylcholine lipids, undergoes a rare nematic $\to$ smectic phase transition upon doping the system with paramagnetic ions (e.g., 2.7 wt % ${\mathrm{Tm}}^{3+}$). In addition, the isotropic phase occurs at a lower temperature than the smectic phase, in contrast to other lyotropic systems and in contrast to the phase behavior predicted by the McMillan model [Phys. Rev. A 4, 1238 (1971)] of smectic ordering.